FIG. 1 illustrates an aircraft powered by counter rotating propellers 6 and 9 of the ultra-high bypass, unducted fan type. The rotational directions are indicated by arrows 12 and 15. FIG. 2 illustrates a type of turbine system which can be used to drive the propellers 6 and 9.
In FIG. 2, fore propeller 9 is attached to a first turbine 18 which rotates in direction 15 as indicated in FIG. 1. The components rotating in this direction are decorated with hatching in FIG. 2. The aft propeller 6 is attached to a second turbine 21 and rotates in direction 12 of FIG. 1. A hot, high-energy gas stream 30 in FIG. 2 provided by a gas generator (not shown) causes rotation of the turbines.
Propeller blades 6 and 9 (which are sometimes called fan blades or propulsor blades in the art) are of the variable pitch type. Variable pitch means that each blade can rotate about a respective pitch axis 6A or 9A in FIG. 2 as indicated by circular arrows 33. A principal reason for changing the pitch is to provide the blades 6 and 9 with the angle of attack which is proper for the present power level of the engine and airspeed of the aircraft.